1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to an active matrix type of liquid crystal display device in which a switching element is disposed in each pixel.
2. Description of the Related Art
Liquid crystal display devices are widely used as display devices for various kinds of image display apparatuses. A typical liquid crystal display device includes liquid crystal compounds having liquid crystal molecules which are sealed in the space between two opposite insulating substrates (hereinafter also referred to simply as substrates) at least one of which is transparent, and electrode-selecting pixels formed on either or both of the inside surfaces (main surfaces) of the two insulating substrates. The liquid crystal display device controls the alignment of the liquid crystal molecules by an electric field formed between the pixels, thereby turning on or off transmitted light or reflected light to display an image.
An active matrix type of liquid crystal display device which controls its image display operation by means of switching elements represented by thin film transistors (TFTs) is widely used as a display device for a display terminal such as a computer because of its advantages of small size, light weight and high image quality.
As described above, the active matrix type of liquid crystal device includes the switching elements disposed in the respective pixels, and has a first electrode (pixel electrode) whose potential is controlled by such a switching element, and a second electrode (counter electrode) whose potential variation is small compared to the first electrode. The active matrix type of liquid crystal display device displays an image by controlling the slate of alignment of the liquid crystal molecules which constitute the liquid crystal compounds by means of an electric field generated between these electrodes.
The active matrix liquid crystal display device is classified into two kinds according to the direction of the lines of electric force which is applied to the liquid crystal molecules sealed between the two substrates by the electrodes.
One of the two kinds is a so-called vertical electric field type which has electrodes disposed opposite to each other with a layer of liquid crystal compounds (hereinafter also referred to simply as a liquid crystal layer) being interposed therebetween, and forms an electric field between these electrodes in a vertical direction with respect to the substrate surfaces (refer to, for example, Japanese Patent Laid-Open No. 257142/1993 as well as the corresponding U.S. Pat. No. 5,432,626).
The other kind is a type which has electrodes shifted in position with respect to the liquid crystal layer, and forms an electric field in a direction parallel to the substrate surfaces (refer to, for example, Japanese Patent No. 2,708,098 and U.S. Pat. No. 5,754,266).
The latter type is called an in-plane switching (IPS) type or a lateral electric field type because the alignment of the liquid crystal molecules depends on the lines of electric force generated in the direction of the substrate surfaces.
In one IPS type of liquid crystal display device, as disclosed in the above-cited U.S. Pat. No. 5,754,266, the aforesaid two kinds of electrodes which generate an electric field to control the alignment of liquid crystal molecules are formed on a substrate (TFT substrate) on which thin film transistors (TFTs) are provided. In another IPS type of liquid crystal display device, as disclosed in U.S. Pat. No. 5,598,285, one of the first and second electrodes is formed on a TFT substrate and the other is formed on the other substrate.
In any of the constructions, each of the first and second electrodes has a region which is not opposite to the other, and care is taken that a conductive thin film is not formed particularly on the main surface of the first electrode that is opposite the region (i.e., a surface opposite to the liquid crystal layer). This art is described in Japanese Patent Laid-Open No. 191994/1995 or the like which is the art of reducing the resistance value of a light shielding film, i.e., a black matrix material.
For such an IPS type of liquid crystal display device, the present inventors formed disconnection testing terminals for gate lines for transmitting control signals to the gate electrodes of thin film transistors (field-effect transistors), by using a conductive material which was called ITO (Indium-Tin-Oxide) in which 1-5 weight % of tin oxide (SnO2) was added to indium oxide (In2O3).
This material has the advantages of less contamination of a liquid crystal layer due to its constituent elements and less deterioration due to oxidation or the like than metal materials used for related-art test terminals. In addition, the present inventors formed data lines each of which supplied a video signal to the corresponding one of the gate lines and to either one of the source and drain electrodes of the corresponding one of the aforesaid transistors, by using a metal material which was higher in conductivity than ITO.
Each of the test terminals made of ITO was formed in such a manner that a hole (or an opening: a contact hole) is formed to extend through at least two kinds of insulating films, i.e., a gate insulating film which covers the gate lines, and a protective film (called a passivation film) which covers the gate insulating film and the data lines, and the at least two kinds of insulating films are kept in contact with the gate line in the exterior of the hole and are led to the upper portion of the protective film.
In addition, a predetermined space must be set between a portion to which is applied a sealing material for forming a region in which to seal the liquid crystal layer and a display region (a region which actually contributes to image display).
Under such conditions, to meet a demand for narrowing a picture frame, i.e., narrowing the area of a peripheral portion relative to the display region, the test terminals made of the above-described ITO were provided in the liquid crystal sealing region. Accordingly, the test terminals made of the above-described ITO are disposed at locations near the liquid crystal layer.
However, when a liquid crystal display device having the test terminal made of such ITO is activated, the following problem (a first subject) occurs: that is to say, unexpected light leaks occur at pixels positioned at an end of the display region, i.e., even during the state of black display, stripes (bright lines) of a display color occur at the pixels positioned at the end of the display region.
In addition, to stabilize the potential of the second electrode (counter electrode), each counter voltage signal line for distributing a voltage to the counter electrodes of the corresponding pixels is required to conduct to a common line (common bus line) disposed at an end of the substrate. Since such counter voltage signal lines are formed on the main surface of the substrate that is opposite to the liquid crystal layer, it is necessary that as in the case of the counter electrodes, a gate insulating film, a protective film or an overcoat film be formed on the counter voltage signal lines, and furthermore, an alignment film be formed at positions where the counter voltage signal lines are in contact with the liquid crystal layer. Accordingly, there is a need for the step of covering the counter voltage signal lines with an insulating film.
However, during the formation of the insulating film particularly by CVD (Chemical Vapor Deposition) using plasma, if electric charges generated in the counter voltage signal lines flow into and are stored in the common bus line during the growth of the insulating film, an unexpected electric discharge occurs between this common bus line and the plasma or a plasma-generating electrode.
If a current which is thought of as due to this electric discharge returns to a counter voltage signal line and the resultant electric power exceeds the allowable limit of the counter voltage signal line, the following problem (a second subject) occurs: that is to say, a material (such as chromium) which constitutes the counter voltage signal line melts and short-circuits to an adjacent line, or causes disconnection of the counter voltage signal line itself.
In addition, it is possible to form the pixel electrodes and the counter electrodes of the IPS type of liquid crystal display device from a material other than the above-described ITO. In this case, portions to be used for evaluating the state of deposition of an insulating layer which constitutes the protective film and an ITO film which constitutes the test terminals are limited to contact holes through which to take out the corresponding test terminals from the gate lines.
However, the shapes of these contact holes are designed to have the same dimensions so that disconnection evaluation of each of the gate lines can be performed under as equal conditions as possible. This leads to the problem (a third problem) that data which is to be referred to in the deposition evaluation cannot be obtained only with such contact holes.
An objective of the present invention is to provide a liquid crystal display device which solves the above-described problems (at least one of the first to third subjects) occurring in a case where at least one of the above-described arts is adopted, i.e., the above-described structures of test terminals, the above-described layout of counter voltage signal lines, or the above-described pixel electrodes and counter electrodes made of a material other than ITO.
The representative ones of the liquid crystal display device structures (the LCD structures) according to the present invention for achieving the aforementioned objective are described with respect to each of the aforementioned subjects as follows.
 less than I. Means for Solving First Subject greater than 
(1) First LCD structurexe2x80x94comprising:
a liquid crystal sealing region being formed by sealing a liquid crystal compound in a space being interposed by first and second substrates which are disposed so that respective main surfaces thereof are opposite to one another;
a display region being defined at the liquid crystal sealing region by an opening of a tight shielding film which is formed of a material having a lower optical transmissivity than that of at least one of the first and second substrates on the at least one of the first and second substrates;
a first conductive layer being formed on at least one of the main surfaces of the first and second substrates in the liquid crystal shielding region so as to be extended from the display region to the exterior thereof;
an insulating layer being formed over the first conductive layer;
a terminal being extended on the insulating layer outside the display region and contacting with the first conductive layer; and
a second conductive layer being formed between the display region and the terminal on the insulating layer,
wherein an electric potential of the second conductive layer is different from that of the first conductive layer.
(2) Second LCD structurexe2x80x94characterized in that the second conductive layer is formed on the insulating layer covering the first layer covering the first conductive layer, and a length of the second conductive layer along the extension direction of the first conductive layer is greater than a length of the terminal along the extension direction of the first conductive layer, in the First LCD structure.
(3) Third LCD structurexe2x80x94characterized in that
the first conductive layers are formed of a plurality of conductive layers being juxtaposed and extended outside the display region, and the liquid crystal display device further comprises a third conductive layer being in contact with the second conductive layer through an opening being formed between the plurality of the first conductive layers, in First LCD structure.
(4) Fourth LCD structurexe2x80x94characterized in that a plurality of pixels each of which has a switching element and a pixel electrode are formed on at least one of the main surfaces of the first substrate and the second substrate, and the first conductive layers are in electrical contact with the switching elements respectively, in Third LCD structure.
(5) Fifth LCD structurexe2x80x94characterized in that the switching element is composed by a semiconductor element having a channel being formed along a direction transverse to the extension direction of the first conductive layer,
a fourth conductive layer is provided on the at least one of the main surfaces of the first and second substrates in the display region and supplies a voltage signal to one end of the channel of the semiconductor element composing the switching element;
the insulating layer is formed of a first insulating layer and a second insulating layer being formed between an upper surface of the first conductive layer and a lower surface of the fourth conductive layer,
another end of the channel of the semiconductor element is in electrical contact with the pixel electrode provided at the pixel, and
the first conductive layer is electrically connected to an electrode for applying an electric field to the channel via the first insulating layer, in Fourth LCD structure.
(6) Sixth LCD structurexe2x80x94characterized in that the third conductive layer is electrically connected with a counter electrode for generating an electric field to be applied to the liquid crystal compound between the counter electrode and the pixel electrode, in Fifth LCD structure.
(7) Seventh LCD structurexe2x80x94characterized in that the terminal and an end of the display region opposite thereto is disposed so as to be spaced from one another with a distance which is four times as long as a pixel region of the pixel along the extension direction of the first conductive layer or longer than the pixel length, in Sixth LCD structure.
(8) Eighth LCD structurexe2x80x94characterized in that the first conductive layer has a portion thereof being with respect to a first extension direction thereof from the display region and extending in a second direction which makes a certain angle to the first extension direction, between a position at which the first conductive layer crosses the second conductive layer and another position the first conductive layer is in contact with the terminal, in Sixth LCD structure.
(9) Ninth LCD structurexe2x80x94characterized in that the first conductive layer has another portion being extended along the first extension direction from an end of the display region being opposite to the terminal to a position at which the portion of the first conductive layer veers in the second extension direction, and a length of the another portion of the first conductive layer along the first extension direction is four times as long as a pixel length of the pixel along the extension direction of the first conductive layer or longer than the pixel length, in Eighth LCD structure.
By adopting any of the above-described first to ninth LCD structures, it is possible to solve the problem that unexpected light leaks occur at pixels positioned at the end of the display region and even during the state of black display, stripes (bright lines) of a display color occur at the pixels positioned at the end of the display region.
 less than II. Means for Solving Second Subject greater than 
(10) Tenth LCD structurexe2x80x94comprising:
a liquid crystal sealing region being formed by sealing a liquid crystal compound in a space being interposed by first and second substrates which are disposed so that respective main surfaces thereof are opposite to one another;
a display region being defined at the liquid crystal sealing region by an opening of a light shielding film which is formed of a material having a lower optical transmissivity than that of at least one of the first and second substrates on the at least one of the first and second substrates;
a plurality of first conductive layers being formed on at least one of the main surfaces of the first and second substrates in the liquid crystal shielding region so as to be extended from the display region to the exterior thereof;
second conductive layer being extended in a direction transverse to extension directions of the first conductive layers at the exterior of the display region and being formed so as to be spaced from the first conductive layer;
an insulating layer being formed over the first conductive layers and the second conductive layer;
a terminal being extended on the insulating layer outside the display region and contacting with the first conductive layer;
a third conductive layer having a portion which is formed on the insulating layer and has electrical contacts with the first conductive layers and the second conductive layer through openings being formed at the insulating layer outside the display region respectively.
(11) Eleventh LCD structurexe2x80x94characterized in that the insulating layer comprises a first insulating layer and a second insulating layer being formed over the first insulating layer,
the second conductive layer has a first portion being formed under the first insulating layer and a second portion being formed between the first insulating layer and the second insulating layer,
the third conductive layer is in electrical contact with the first portion through openings being formed at the first insulating layer and is in electrical contact with the second portion through an opening being formed at the second insulating layer, in Tenth LCD structure.
(12) Twelfth LCD structurexe2x80x94characterized in that a terminal is provided for the second conductive layer, and the terminal is formed of a conductive material being in electrical contact with the first portion through openings being formed at the first insulating layer and the second insulating layer and being also in electrical contact with the second portion through an opening being formed at the second insulating layer, in Eleventh LCD structure.
(13) Thirteenth LCD structurexe2x80x94characterized in that a fourth conductive layer extending in a direction transverse to the extension direction of the first conductive layer is provided on the insulating layer in an area between the display region and the second conductive layer, and
the fourth conductive layer is in electrical contact with the plurality of first conductive layers through respective openings being formed at the insulating layer, in Eleventh LCD structure.
(14) Fourteenth LCD structurexe2x80x94characterized in that the insulating layer comprises a first insulating layer and a second insulating layer being formed over the first insulating layer, the first conductive layer is separated into a separated portion thereof and another portion thereof between a first position at which the first conductive layer contacts the third conductive layer and a second position at which the first conductive layer contacts with the fourth conductive layer,
the separated portion of the first conductive layer is formed between the first insulating film and the second insulating layer, while the another portion thereof is extended under the first insulating layer from the second position toward the display region, and
the separated portion of the first conductive layer is electrically connected to the another portion thereof by contacting the fourth conductive layer through respective openings being formed at the first and second insulating layers, in Thirteenth LCD structure.
(15) Fifteenth LCD structurexe2x80x94characterized in that a region where the fourth conductive layer is contact with the first conductive layer has a length along the extension direction of the first conductive layer being half as long as a length of the fourth conductive layer along the extension direction of the first conductive layer or longer than the length of the fourth conductive layer, in Thirteenth LCD structure.
(16) Sixteenth LCD structurexe2x80x94characterized in that respective ends of the plurality of first conductive layers being opposite to the second conductive layer are electrically connected to each other by a conductive layer being extended in a direction transverse to the extension direction of the first conductive layers, in Thirteenth LCD structure.
(17) Seventeenth LCD structurexe2x80x94characterized in that the conductive layer being connected to the ends of the plurality of first conductive layers has a length thereof along the extension direction of one of the first conductive layers being smaller than a length of the second conducive layer along the extension direction of one of the first conductive layer, in Fifteenth LCD structure.
(18) Eighteenth LCD structurexe2x80x94characterized in that the opening of the insulating layer through which the second conductive layer and the third conductive layer contacts with one another is extended along the extension direction of the fourth conductive layer extending in a direction transverse to the extension direction of the second conductive layer, and an area where the first conductive layer and the third conductive layer contacts with one another has a length thereof along the extension direction of the first conductive layers being greater than a length of an area where the second conductive layer and the third conductive layer contact with one another along the extension direction of the first conductive layers, in Seventeenth LCD structure.
(19) Nineteenth LCD structurexe2x80x94characterized in that the third and fourth conductive layers are formed of an oxide material lie an ITO, or SnO2.
By adopting any of the tenth to nineteenth LCD structures, it is possible to the problem that during the insulating layer growth by CVD using plasma (Chemical Vapor Deposition) using plasma, an unexpected electric discharge occurs between the common line and the plasma or an electrode for generating the plasma owing to the flow of electric charges generated in counter voltage signal lines into the common line. It is also possible to the problem that if a current due to such an electric discharge returns to a counter voltage signal line, a material which constitutes the counter voltage signal line melts and the counter voltage signal line short-circuits to an adjacent line, or the counter voltage signal line itself is disconnected.
 less than Ill. Means for Solving Third Subject greater than 
(20) Twentieth LCD structurexe2x80x94comprising:
a liquid crystal sealing region being formed by sealing a liquid crystal compound in a space being interposed by first and second substrates which are disposed so that respective main surfaces thereof are opposite to one another;
a display region being defined at the liquid crystal sealing region by an opening of a light shielding film which is formed of a material having a lower optical transmissivity than that of at least one of the first and second substrates on the at least one of the first and second substrates;
a plurality of first conductive layers being juxtaposed in the display region and extended along a first direction,
a first insulating layer being formed over the first conductive layers and the second conductive layers;
a plurality of second conductive layers being juxtaposed over the first insulating layer in the display region and extended along a second direction transverse to the first direction;
a pixel being disposed in a region surrounded by a pair of the first conductive layers and a pair of the second conductive layers in the display region;
a second insulating layer being formed over the second conducive layers; and
a third conductive layer having a portion being formed on the second insulating layer at the exterior of the display region;
wherein at least one of the first conductive layers and the second conductive layers is extended to the exterior of the display region and is contacted with the portion of the third conductive layer through an opening formed through the first and second insulating layers at the exterior of the display region,
wherein a fourth conductive layer is formed under the first insulating layer and a fifth conductive layer is formed between the first and second insulating layers respectively at the exterior of the display region,
wherein a first opening down to an upper surface of the fourth conductive layer is provided in the first and second insulating layers, a second opening down to an upper surface of the fifth conductive layer is provided in the second insulating layer so as to be spaced from the first opening, and a sixth conductive layer is formed on the second insulating layer so as to surround the first opening and the second opening and so as to extend into insides of the first opening and the second opening,
wherein the third conductive layer and the sixth conductive layer are formed of a conductive material having optical transmissivity higher than that of the fourth conductive layer and the fifth conductive layer.
(21) Twenty-First LCD structurexe2x80x94characterized in that the sixth conductive layer being formed at the first opening and the sixth conductive layer being formed at the second opening are spaced from one another on the second insulating layer, in the
(22) Twenty-Second LCD structurexe2x80x94characterized in that both the fourth conductive layer and the fifth conductive layer are electrically isolated (insulated) from the first, second and third conductive layers, in Twentieth LCD structure.
(23) Twenty-Third LCD structurexe2x80x94characterized in that the third and sixth conductive layers are formed of an oxide material like an ITO, or SnO2, in Twentieth LCD structure.
By adopting any of the twentieth to twenty-third LCD structures, disconnection evaluation of each gate line is enabled.
In addition, the following structures are available as combinations of the structures which have been described above as the means for solving any of the first to third subjects.
(24) Twenty-Fourth LCD structurexe2x80x94characterized in that the second conductive layer in the means for solving the first subject is used also as the fourth embodiment in the means for solving the second subject.
(25) Twenty-Fifth LCD structurexe2x80x94characterized in that the terminal in the means for solving the first subject is provided on a side nearer to the display region than is the second conductive layer in the means for solving the second subject, and is connected to ends of the plurality of first conductive layers in the means for solving the second subject and is provided on a side nearer to the display region than is the conductive layer extended along the second conductive layer.
(26) Another LCD structurexe2x80x94characterized in that the display region in the means for solving the first subject is defined by a side, nearer to the display region, of the one of the fourth conductive layers that is disposed nearest to the end of the display region, or by an edge of the opening formed in the light shielding material, the edge being nearest to the end of the display region.
(27) Another LCD structurexe2x80x94characterized in that the first and second openings in the means for solving the third subject are provided at a position opposite to the end of the second conductive layer in the means for solving the first subject or the fourth conductive layer in the means for solving the second subject.
Incidentally, the present invention is not limited to any of the above-described structures and the structures of embodiments which will be described below, and it goes without saying that various modifications can be made without departing from the technical ideas of the present invention.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.